Pick and place machines are generally used to manufacture electronic circuit boards. A blank printed circuit board is usually supplied to the pick and place machine, which then picks electronic components from component feeders, and places such components upon the board. The components are held upon the board temporarily by solder paste, or other adhesive until a subsequent step in which the solder paste is melted, or the adhesive is fully cured.
Pick and place machine operation is challenging. Since machine speed corresponds with throughput, the faster the pick and place machine runs, the less costly the manufactured board will be. Additionally, placement accuracy is extremely important. Many electrical components, such as chip capacitors and chip resistors are relatively small and must be accurately placed on equally small placement locations. Other components, while larger, have a significant number of leads or conductors that are spaced from one another at a relatively fine pitch. Such components must also be accurately placed to ensure that each lead is placed upon the proper pad. Thus, not only must the machine operate extremely fast, but it must also place components extremely accurately.
Picking up a component requires the placement head to be positioned over the pick up point for the target component. Once the nozzle is so positioned, it is lowered to a point just above the component and a vacuum is applied through the nozzle which sucks the component up and temporarily attaches it to the end of the nozzle. Each component is positioned at its pick point by a component feeder mechanism. Typical feeder mechanisms include tape feeders, vibratory feeders and tray feeders. Once a component is picked up by the nozzle, the feeder mechanism must move another component into the pick position. If the component pick operation is not successful, defective work pieces are produced. Defects on work pieces that are known to be caused by bad pick operations are tombstoned components, missing components, wrong components, wrong component polarity, and misplaced components. Further, defects are also caused by operators loading feeders into incorrect positions; allowing feeders to run out of components; defective or broken feeders, component tapes and/or nozzles; incorrectly programmed nozzle pick heights; and incorrectly positioned components.
Providing a method of assessing the effectiveness of a pick operation without slowing machine throughput, and without adding significant hardware to the pick and place machine, would allow pick and place machine operation to more accurately identify pick errors, and ensure that such errors do not generate defective workpieces. Additionally, identifying pick errors before they create defective work pieces (such as entire circuit boards) would vastly simplify rework since the pick and place machine can simply discard the erroneously picked component and pick another one. In contrast, if the erroneously picked component is actually placed upon the workpiece, the error may not be discovered until after final soldering or adhesive curing is complete. Given that a particular workpiece, such as a circuit board, may contain hundreds or even thousands of individual components, the workpiece itself could be relatively costly, and require relatively costly manual rework in order to ensure that the entire workpiece is not lost.